Transducers are useful in inertial guidance systems to sense the acceleration or orientation of a device, such as a spacecraft, aircraft, land-based vehicle, or handheld device. They are also used in vehicles to sense impacts and deploy various devices to protect the passengers (for example, air bags in automobiles). Transducers may be required to sense acceleration or other phenomena along one, two, or three axes or directions. From this information, the movement or orientation of the device can be ascertained.
In the ongoing effort to reduce the size and cost of transducers, a variety of sensing devices have been proposed. Many of these devices, especially those which function as accelerometers, include capacitive structures, some of which are constructed using semiconductor manufacturing techniques and/or bulk micromachining. The capacitive structures generally consist of at least one conductive plate, formed of doped silicon or the like, which is mounted on a substrate by way of a compliant suspension. The plate is positioned parallel to a planar surface of the substrate and forms capacitances with fixed structures mounted on the substrate. When the plate moves due to acceleration, the capacitances between the plate and these fixed structures changes. These changes are then sensed by the electronic circuitry of the device and are converted to signals representative of the acceleration.
FIGS. 1 and 2 illustrate one example of a prior art capacitive accelerometer 111 of the type described above. The accelerometer, which is adapted to sense Z axis accelerations (that is, accelerations perpendicular to the major planar surface of the device) comprises a proof mass plate 113 that is suspended by torsional spring flexures 115 in a see-saw type configuration. This proof mass is “unbalanced” because of the extra mass present on the “heavy” end 116. The proof mass plate is supported by a central anchor 121 which is attached to a substrate 123 (see FIG. 2). When the device experiences an acceleration along the Z-axis, the proof mass rotates (or tips) around the flexure axis defined by the suspension. The resulting motion, which is proportional to the amount of acceleration the device is experiencing, is measured by capacitive sensor plates 117, 119 disposed beneath the proof mass, and is converted into electronic signals representative of the acceleration. The use of an unbalanced proof mass in this device is required, because a balanced proof mass would not tilt when subjected to Z axis acceleration, and hence would not properly sense the acceleration.
The device shown in FIGS. 1 and 2 is a single axis accelerometer which senses acceleration only along the Z axis. However, accelerometers are also known to the art which are adapted to sense accelerations along multiple axes. One such device is shown in FIG. 3. The device illustrated therein is an XY displacement accelerometer 231 that is adapted to sense accelerations along two orthogonal in-plane axes of the device. The accelerometer includes a proof mass plate 233 that comprises a central mass 235 attached to a frame 237 by way of first 241 and second 243 sets of sense fingers. The first and second sets of sense fingers are disposed in a mutually perpendicular arrangement. The frame is supported by a plurality of anchors 245 to which it is attached by a series of compliant springs 247. In structures of this type that are designed to sense a similar magnitude acceleration along the two orthogonal sense axes, the springs 247 will have similar stiffness in the X and Y directions.
In FIG. 3, each sense finger 241 is surrounded by two sets of fixed fingers 251 and 252, respectively. Similarly, each sense finger 243 is surrounded by two sets of fixed fingers 253 and 254, respectively, which are anchored to the substrate. When the proof mass moves in the Y direction, the capacitances between the moving fingers 241 and the fixed fingers 251, 252 change. Similarly, when the proof mass moves in the X direction, the capacitances between the moving fingers 243 and the fixed fingers 253, 254 change. The device is provided with electronic circuitry which converts these capacitive changes to signals representative of the acceleration along the X and Y axes.
The accelerometers described above are manufactured as single axis or dual axis devices. However, some applications require the ability to sense acceleration along three mutually orthogonal axes. This has been achieved, in some instances, by utilizing three accelerometers of the type depicted in FIG. 1, and positioning them such that their sensitive axes are disposed in a mutually orthogonal configuration. However, the final package resulting from such a configuration is inherently bulky, since each accelerometer must be positioned such that its sense axis is orthogonal to the sense axes of the other accelerometers. Hence, the entire package can never be formed as a single planar device.
Some transducers have also been proposed which are adapted to sense accelerations along three axes, and which are more compact than the three-axis accelerometer designs described above. An example of such a transducer is shown in FIGS. 7 and 8. In the construction depicted therein, the middle layer 32 typically serves as a single proof mass equipped with capacitive plates 30 for sensing accelerations along the X and Y axes. The devices of FIGS. 7 and 8 are similar in some respects to the devices of FIG. 3, but have additional plates 40 and 43 disposed above the proof mass 32, as well as a series of conductors 23, 24, 26 and 27 mounted on the substrate 11 and disposed below the proof mass that are capacitively coupled to the proof mass. Accelerations along the Z axis (that is, the direction orthogonal to the major surfaces of the substrate) are sensed in this type of device by measuring the differential capacitance between the proof mass 30 and the plates above, 40–43, vs. below, 23, 24, 26 and 27. Accelerations in the x-y plane (that is, the plane of the major surfaces of the substrate) are sensed by measuring differential overlap capacitances between the proof mass and the appropriate plates below it (i.e., 13 and 14 vs. 18 and 19, 16 and 17 vs. 21 and 22).
While the transducers shown in FIGS. 7 and 8 are a notable advance in the art and do achieve reductions in the size of the overall package, their construction requires a “three layer” process flow. This phrase refers to the number of layers of conductive material (typically doped polysilicon) used to fabricate the sense capacitor plates present in the device or required by the manufacturing process. In particular, the devices of FIGS. 7 and 8 require one conductive layer each for the proof mass 32, the series of conductors 23, 24, 26 and 27, and the plates 40 and 41. By contrast, the accelerometers depicted in FIGS. 1 and 2 may be fabricated by a “two layer” process flow. In particular, in those devices, the bottom plates 117, 119 are formed from a first conductive layer, and the proof mass 113 is formed from a second conductive layer. Compared to a two layer process flow, a three layer process flow is less desirable in that it requires at least two additional masking steps. Consequently, devices manufactured by a three layer process are generally more expensive to manufacture, and have a lower yield, than comparable devices manufactured via a two layer process.
Transducers made by a three layer process flow are also typically more susceptible to packaging stresses and conductive layer stress gradients than those made by two layer process flows. These effects are particularly problematic for the Z axis output of the device. This can be appreciated from FIG. 9, which depicts a general three layer device 121 containing top 123, middle 125 and bottom 127 layers supported on a substrate 129. The middle layer 125 and top layer 123 are supported-several microns above the substrate 129, and thus tends to deform along the Z axis when the device is subjected to strain along the in-plane axes of the substrate. Such strain is commonly encountered, for example, during thermal cycling. In addition, stress gradients in the conductive layers 123 and 125 also contribute to the deformation (curvature) of these layers. The deformation of the top layer modifies the capacitance 131 between the top and middle layers, and hence changes the Z axis output of the device. Packaging stresses can be reduced by coating the transducer die with an elastomer (sometimes referred to as a “dome coat”). However, such coatings complicate the manufacturing process.
There is thus a need in the art for a compact, single die transducer that can sense along multiple axes, that can be fabricated as a planar structure in a two layer process flow, that is less susceptible to package stress and conductive layer stress gradients, and that does not require the use of a dome coat or other features designed to reduce packaging stress.